1. Field of the Invention
The present invention relates to a heat-assisted magnetic recording head for use in heat-assisted magnetic recording where a magnetic recording medium is irradiated with near-field light to lower the coercivity of the magnetic recording medium for data recording, a method of manufacturing the same, and a head gimbal assembly and a magnetic recording device each of which includes the heat-assisted magnetic recording head.
2. Description of the Related Art
Recently, magnetic recording devices such as a magnetic disk drive have been improved in recording density, and thin-film magnetic heads and magnetic recording media of improved performance have been demanded accordingly. Among the thin-film magnetic heads, a composite thin-film magnetic head has been used widely. The composite thin-film magnetic head has such a structure that a reproducing head including a magnetoresistive element (hereinafter, also referred to as MR element) intended for reading and a recording head including an induction-type electromagnetic transducer intended for writing are stacked on a substrate. In a magnetic disk drive, the thin-film magnetic head is mounted on a slider that flies slightly above the surface of the magnetic recording medium.
Magnetic recording media are discrete media each made of an aggregate of magnetic fine particles, each magnetic fine particle forming a single-domain structure. A single recording bit of a magnetic recording medium is composed of a plurality of magnetic fine particles. For improved recording density, it is necessary to reduce asperities at the borders between adjoining recording bits. To achieve this, the magnetic fine particles must be made smaller. However, making the magnetic fine particles smaller causes the problem that the thermal stability of magnetization of the magnetic fine particles decreases with decreasing volume of the magnetic fine particles. To solve this problem, it is effective to increase the anisotropic energy of the magnetic fine particles. However, increasing the anisotropic energy of the magnetic fine particles leads to an increase in coercivity of the magnetic recording medium, and this makes it difficult to perform data recording with existing magnetic heads.
To solve the foregoing problems, there has been proposed a technique so-called heat-assisted magnetic recording. This technique uses a magnetic recording medium having high coercivity. When recording data, a magnetic field and heat are simultaneously applied to the area of the magnetic recording medium where to record data, so that the area rises in temperature and drops in coercivity for data recording. Hereinafter, a magnetic head for use in heat-assisted magnetic recording will be referred to as a heat-assisted magnetic recording head.
In heat-assisted magnetic recording, near-field light is typically used as a means for applying heat to the magnetic recording medium. A commonly known method for generating near-field light is to use a near-field optical probe or so-called plasmon antenna, which is a piece of metal that generates near-field light from plasmons excited by irradiation with light.
However, a plasmon antenna that is directly irradiated with light to generate near-field light is known to exhibit very low efficiency of conversion of the applied light into near-field light. The energy of the light applied to the plasmon antenna is mostly reflected off the surface of the plasmon antenna, or converted into thermal energy and absorbed by the plasmon antenna. The plasmon antenna is small in volume since the size of the plasmon antenna is set to be smaller than or equal to the wavelength of the light. The plasmon antenna therefore shows a significant increase in temperature when it absorbes the thermal energy.
Such a temperature increase makes the plasmon antenna expand in volume and protrude from a medium facing surface, which is the surface of the heat-assisted magnetic recording head to face the magnetic recording medium. This causes an end of the reproducing head located in the medium facing surface to get farther from the magnetic recording medium, thereby causing the problem that a servo signal cannot be read during recording operations.
To cope with this, as described in, for example, U.S. Pat. No. 7,330,404, there has been proposed a technique in which light propagating through a waveguide is not directly applied to a plasmon antenna but is coupled with a near-field light generating element via a buffer layer in a surface plasmon polariton mode to thereby excite surface plasmons on the near field light generating element. The near-field light generating element has a near-field light generating part that is located in the medium facing surface and generates near-field light. According to this technique, the light propagating through the waveguide is totally reflected at the interface between the waveguide and the buffer layer to generate evanescent light permeating into the buffer layer. The evanescent light and collective oscillations of charges on the near-field light generating element, i.e., surface plasmons, are coupled with each other to excite the surface plasmons on the near-field light generating element. In the near-field light generating element, the excited surface plasmons propagate to the near-field light generating part, and near-field light occurs from the near-field light generating part. According to this technique, since the near-field light generating element is not directly irradiated with the light propagating through the waveguide, it is possible to prevent an excessive increase in temperature of the near-field light generating element.
In a near-field light generating device including the waveguide, the buffer layer and the near-field light generating element, a possible layout of the waveguide, the buffer layer and the near-field light generating element is such that the near-field light generating element is opposed to the top or bottom surface of the waveguide with the buffer layer interposed therebetween in the vicinity of the medium facing surface. The waveguide allows propagation of laser light to be used for generating near-field light. In order to generate surface plasmons of high intensity on the near-field light generating element, the foregoing layout requires that the laser light to propagate through the waveguide be TM-polarized light whose electric field oscillates in a direction perpendicular to the top and bottom surfaces of the waveguide.
Possible techniques of placement of a light source that emits the laser light to propagate through the waveguide are broadly classified into the following two. A first technique is to place the light source away from the slider. A second technique is to fix the light source to the slider.
The first technique is described in JP 2007-200475 A, for example. The second technique is described in U.S. Patent Application Publication No. 2008/0002298 A1 and U.S. Patent Application Publication No. 2008/0043360 A1, for example.
The first technique requires an optical path of extended length including such optical elements as a mirror, lens, and optical fiber in order to guide the light from the light source to the waveguide. This causes the problem of increasing energy loss of the light in the path. The second technique is free from the foregoing problem since the optical path for guiding the light from the light source to the waveguide is short.
The second technique, however, has the following problem. Hereinafter, the problem that can occur with the second technique will be described in detail. The second technique typically uses a laser diode as the light source. The laser diodes available include edge-emitting laser diodes and surface-emitting laser diodes. In an edge-emitting laser diode, the emission part for emitting the laser light is located in an end face that lies at an end of the laser diode in a direction parallel to the plane of an active layer. The emission part emits the laser light in the direction parallel to the plane of the active layer. In a surface-emitting laser diode, the emission part for emitting the laser light is located in a surface that lies at an end of the laser diode in a direction perpendicular to the plane of the active layer. The emission part emits the laser light in the direction perpendicular to the plane of the active layer.
The laser light emitted from a laser diode can be made incident on the waveguide by a technique described in U.S. Patent Application Publication No. 2008/0002298 A1, for example. This publication describes arranging a surface-emitting laser diode with its emission part opposed to the surface of the slider on the trailing side so that the laser light emitted from the emission part is incident on the waveguide from above. Surface-emitting laser diodes, however, typically have a lower optical output as compared with edge-emitting laser diodes. The technique therefore has the problem that it is difficult to provide laser light of sufficiently high intensity for use in generating near-field light.
The laser light emitted from a laser diode may be made incident on the waveguide by other techniques. For example, U.S. Patent Application Publication No. 2008/0043360 A1 describes a technique in which the incident end face of the waveguide is arranged at the surface opposite to the medium facing surface of the slider, and the laser diode is arranged with its emission part opposed to this incident end face so that the laser light emitted from the emission part is incident on the incident end face of the waveguide without the intervention of any optical element. This technique allows the use of an edge-emitting laser diode which has a high optical output. However, this technique has the problem that it is difficult to align the emission part of the laser diode with respect to the incident end face of the waveguide with high precision, since the position of the emission part of the laser diode can vary within a plane perpendicular to the optical axis of the waveguide.
To cope with this, the edge-emitting laser diode may be fixed to the top surface of the slider that lies at an end of the slider above the top surface of the substrate, so that the laser light is emitted in a direction parallel to the top surface of the slider, while arranging the waveguide so that the incident end face of the waveguide is opposed to the emission part of the laser diode.
An edge-emitting laser diode typically includes: an n-substrate having two surfaces that face toward mutually opposite directions; an n-electrode bonded to one of the two surfaces of the n-substrate; a laser structure part integrated on the other of the two surfaces of the n-substrate; and a p-electrode bonded to the laser structure part such that the laser structure part is sandwiched between the n-substrate and the p-electrode. The laser structure part includes the active layer. The edge-emitting laser diode has two surfaces that lie at opposite ends in the direction perpendicular to the plane of the active layer; one is formed by the surface of the n-electrode, and the other is formed by the surface of the p-electrode. The distance between the surface of the p-electrode and the active layer is smaller than the distance between the surface of the n-electrode and the active layer. When the edge-emitting laser diode is fixed to the top surface of the slider as mentioned above, the laser diode is preferably arranged so that the surface of the p-electrode closer to the active layer faces the top surface of the slider, rather than so that the surface of the n-electrode faces the top surface of the slider. The reason is that the former arrangement can reduce the difference in level between the bottom surface of the laser diode and the position of the incident end face of the waveguide. One of possible methods for fixing the edge-emitting laser diode to the slider so that the surface of the p-electrode faces the top surface of the slider is to solder-bond the p-electrode of the laser diode to a conductive layer that is disposed on the top surface of the slider.
Now, let us consider a configuration in which the edge-emitting laser diode is fixed to the slider so that the surface of the p-electrode faces the top surface of the slider, and the waveguide is arranged so that the incident end face of the waveguide is opposed to the emission part of the laser diode as described above. Here, suppose also that the near-field light generating element is arranged in the vicinity of the medium facing surface so that the near-field light generating element is opposed to the top surface or the bottom surface of the waveguide with the buffer layer interposed therebetween, as mentioned previously. This configuration requires that the laser light to propagate through the waveguide be TM-polarized light as mentioned previously. It is therefore necessary to use an edge-emitting laser diode that emits polarized light of TM mode whose electric field oscillates in the direction perpendicular to the plane of the active layer. Such a configuration has the following problem.
A laser diode that emits polarized light of TM mode is typically achieved by a strained quantum well structure in which the active layer is given a tensile strain. In the case of the laser diode that emits polarized light of TM mode, the state of polarization of the emitted light changes more sensitively in response to the stress on the active layer than in the case of a laser diode that emits polarized light of TE mode whose electric field oscillates in the direction parallel to the plane of the active layer. Consequently, if the p-electrode of the laser diode that emits polarized light of TM mode is solder-bonded to the conductive layer disposed on the top surface of the slider, a stress occurring from the solidification of the solder may act on the active layer located near the p-electrode to change the state of polarization of the emitted light.